Ventricular tachyarrhythmias, in which the ventricles beat more rapidly and irregularly than normal, can be due to a variety of etiologies. Certain patients, for example, are prone to premature ventricular contractions due to ectopic excitatory foci in the ventricular myocardium. Another cause of ventricular tachyarrhythmia is atrial fibrillation where the atria depolarize in a chaotic fashion with no effective pumping action. The intrinsic ventricular rhythm that occurs during an episode of atrial fibrillation is a result of the chaotically occurring depolarizations occurring in the atria being passed through the AV node to the ventricles. The intrinsic ventricular rate is thus governed by the cycle length of the atrial fibrillation and the refractory period of the AV node. Although the intrinsic ventricular rate is less than the atrial rate, due to the refractory period of the AV node, it is still rapid and irregular.
When the ventricles contract at irregular intervals, the contraction can occur prematurely before diastolic filling is complete which decreases the stroke volume for that contraction. This can be especially significant in, for example, congestive heart failure patients who are already hemodynamically compromised. Concomitant atrial fibrillation where the atria no longer act as effective priming pumps can also contribute to the problem. An irregular ventricular rate can thus depress cardiac output and cause such symptoms as dyspnea, fatigue, vertigo, and angina. An objective of the present invention is to use pacing therapy to maintain hemodynamic stability in the presence of an irregular intrinsic ventricular rhythm.